Capacitive-discharge ignition system



Aprilv7, 1970 1.. A. CHAVIS CAPACITIVE-DISCHARGE IGNITION SYSTEM 7 6 9 1 8 2 m A d e l 1 F 8 mm w MN N 8 N iv g N E N@ M(E mm N\ F S N om mm N mm mm I!!! o ATTORNEYS United States Patent O 3,504,658 CAPACITIVE-DISCHARGE IGNITION SYSTEM Leon A. Chavis, Detroit, Mich., assignor to Mallory Electric Corporation, Detroit, Mich., a corporation of Michigan Filed Aug. 28, 1967, Ser. No. 663,716 Int. Cl. F02p 1/00; H05b 37/02, 39/04 US. Cl. 123148 2 Claims ABSTRACT OF THE DISCLOSURE The capacitive-discharge ignition system utilizes a capacitor to develop the high voltage necessary to fire a vehicle spark plug. The capacitor is placed in series with the primary winding of a step-up transformer. A silicone controlled rectifier is provided in parallel with the capacitor. The capacitor is initially charged. When the silicone controlled rectifier is triggered to conduct, it causes the capacitor to discharge through the primary of the transformer. The discharge is sudden and results in inducing a voltage in the secondary winding sufiicient to fire a spark plug.

After the capacitor has discharged, the magnetic field about the primary winding of the transformer collapses. The collapsing field recharges the capacitor in the opposite polarity. A diode is provided in parallel with the primary winding of the transformer. The diode forms part of a circuit which permits the capacitor to discharge in the opposite direction. The capacitor does not discharge through the primary winding of the transformer. After the capacitor has again discharged, the field about the primary winding collapses to charge the capacitor to the original polarity. The cycle is then repeated.

BACKGROUND OF THE INVENTION In conventional inductive-discharge ignition systems, the coil develops the high voltage necessary to cause the vehicle spark plugs to fire. Capacitive-discharge ignition systems have previously been proposed. Capacitive-discharge systems have several advantages over conventional systems. One advantage is that such a system has the ability to cause firing of defective spark-plugs. Additionally, in such systems the capacitor is not discharged as often when operating at low speeds as it is when operating at high speeds. This allows a relatively long time in which the capacitor achieves a full charge. This is advantageous in that it results in improved cold-weather starting and reduces power consumption by the ignition system while starting.

The present invention provides an improved version of a capacitativedischarge ignition system. In accordance with the present invention, a diode is provided to permit discharge of the capacitor when it is charged oppositely to its original charge. The diode shunts the discharge around the primary of the transformer. This prevents bucking currents in the transformer and provides an exponential decay of the transformer field. This is advantageous in that it avoids wide and erratic excursions during decay of the field.

SUMMARY OF THE INVENTION The capacitive-discharge ignition system for an internal combustion engine includes a step-up transformer having its primary winding in series with a capacitor. The capacitor is charged in one polarity by a DC power source. A solid-state controlled rectifier is provided to discharge the capacitor through the primary winding in timed sequence with an operating engine to induce a 3,504,658 Patented Apr. 7, 1970 sequence of collapse of the magnetic field in the transformer.

In the drawing:

The figure is a schematic illustration of a capacitivedischarge ignition system forming an embodiment of the present invention.

Referring to the figure, it will be noted that a step-up transformer 10 is included in the circuit for providing a voltage sufiiciently high to fire a spark plug of a vehicle engine. The transformer 10 comprises a primary winding 12 and a secondary winding 14. The high voltage output of the winding 14 is utilized for firing the vehicle spark plugs.

A lead 16 extends from one side of the primary winding 12 and is connected to a ground lead 18. A lead 20 extends from the other side of the primary winding and is connected to one side of a capacitor 22. A lead 24 extends from the other side of the capacitor 22 and is connected to a DC power source which is capable of supplying a relatively high voltage, in the neighborhood of 250 volts.

In operation of the system, the capacitor 22 is first charged to the voltage of the power supply. The capacitor is then suddenly discharged through the primary winding 12 whereby the voltage applied to the primary winding rises to the full voltage of the capacitor, in the present instance about 250 volts, in a very short period of time, for example, two micro-seconds. The voltage induced on the secondary winding 14 is suflicient to have the spark plugs fire.

The power supply for the circuit includes a vehicle battery 26. The negative side of the vehicle battery is grounded at 29. A lead 30 extends from the positive side of the battery to an ignition switch 32. The other side of the ignition switch 32 is connected to an oscillator 34 by a lead 37. The oscillator 34 may be one of several commercially available oscillators. One preferred oscillator is disclosed in my co-pending application, Ser. No. 669,159 filed Sept. 20, 1967.

The output of the oscillator is connected to one side of a primary winding 36 of a step;up transformer 38. The other side of winding 36 is connected to ground at 40. One side of the secondary winding 42 is connected to ground lead 18 by a lead 44. The lead 24 extends from the other side of the secondary winding 42.

A buffer capacitor 46 is provided across the secondary winding 42 'by means of lead 48. A rectifier 50- is provided for lead 24 to rectify the output of the transformer 38. The output of the rectifier is filtered by means of an inductance 52 provided in lead 24 and a capacitor 54 provided in lead 56 which extends from lead 24 to the ground lead 18.

As will be appreciated, the oscillator 34 causes a pulsating voltage in the primary winding 36 which results in a stepped-up voltage being induced in the secondary winding 42. The filter and rectifier components cause a DC voltage of higher value than the battery voltage to be present for charging the capacitor 22. As previously mentioned, one suitable voltage is 250 volts. A hold-off diode 58 is provided between the inductance 52 and capacitor 22. The function of diode 58 is to isolate these circuits and prevent reverse polarity voltage from being applied to the voltage amplifier stage.

A solid-state controlled rectifier '60 is provided in lead 62 which extends between lead 24' and lead 18. The rectifier, which may be a silicone controlled rectifier, has an anode 64, a cathode 66 and a gate 68. As is well known, a controlled rectifier is a solid state four-layer device. In its normal state, the controlled rectifier acts as an open circuit that will not pass current. When an appropriate voltage or current pulse is applied to the gate electrode, it will cause the controlled rectifier to be forward biased to permit current flow. Application of the proper polarity voltage to the controlled rectifier, will allow electrons to flow from the cathode to the anode. Reversal of the voltage polarity results in the controlled rectifier being an open circuit. Similarly, when the controlled rectifier is conducting, application of a reverse polarity to the gate electrode will place the controlled rectifier in its original state of an open circuit. Thus, the controlled rectifier can act as a controlled switching di ode capable of being switched on or off by application of voltages of appropriate polarity.

In the present embodiment, the gate 68 is connected to a pulse source 70. The pulse source 70 may be, for example, the usual breaker points which are opened and closed in timed relation to the speed of the distributor shaft. However, the signal may be derived from any suitable source from the engine. One preferred method for providing a pulse is disclosed in my co-pending application Ser. No. 447,004, filed Apr. 9, 1965.

The circuit is completed by a pair of diodes 72, 74 and an L-C circuit therebetween. A lead 76 extends from the lead 24, from a point between the diode 58 and capacitor 22 into connection with the ground lead 18. The diode 72 and parallel inductance 77 and capacitor 79 are provided in lead 76. A lead 78 extends from a point between the capacitor 22 and one side of primary Winding 12 into connection with a lead 16 on the other side of the primary winding 12.

It will be noted from the circuit that when the capacitor 22 is charged plus-to-minus as illustrated, one possible path for electron flow for discharge of the ca pacitor is from the negative side of the capacitor through the primary winding 12, thence through the controlled rectifier 60 to the positive side of the capacitor. When the polarity of the capacitor is reversed, the capacitor may discharge by electron flow through the diode 72 and diode 74. It will be noted that the diode 74 functions as a virtual short circuit across the primary winding 12.

Operation of the circuit may now be understood. With the ignition switch '32 closed to apply battery voltage to the system, the oscillator 34 oscillates providing a stepped-up voltage to the secondary winding 42 of the transformer 38. This voltage is rectified and filtered. It reaches an amplitude of about 250 volts. Capacitor 22 is charged to this value.

When a signal is applied to the gate 68 of the controlled rectifier 60, the controlled rectifier is turned on and current flows through the anode-cathode circuit thereof. When the controlled rectifier conducts, it acts as a virtual short-circuit causing the capacitor 22 to rapidly discharge through the primary winding 12 of the trans former 10.

The voltage applied to the primary winding 12 of the transformer rises to the full capacitor voltage in a very short period of time, from one to three micro-seconds. A voltage is induced in the secondary winding 14 sufficient to cause firing of a vehicle spark plug.

After the capacitor 22 has discharged, the magnetic field about the primary winding 12 collapses. This field collapses at a slower rate than the rise time. The col lapsing field recharges the capacitor 22 but in the op posite polarity, that is, instead of the plus-tO-minus po larity illustrated, the capacitor assumes a minus-to-plus polarity. The capacitor 22 then discharges through diodes 74, 72. This discharge path is of importance in the invention in that the discharge is not through the primary winding 12 of the transformer 10. This permits the field in the transformer to decay exponentially and prevents bucking currents in the transformer. Bucking currents in the trapsformer are undesirable because they result in non-uniform jagged voltage excursions which are detrimental to the eificiency of operation of the ignition systern.

As the capacitor 22 again discharges to zero, a field is built in the inductance 77. When the capacitor has dicharged completely, the field about the inductance 77 collapses causing the capacitor 22 to be charged to the original polarity. The capacitor 79 causes the L-C circuit to be in resonance. The capacitor is fully recharged by the power supply to the original value. Conduction of the controlled rectifier is discontinued by the reverse voltage applied by the anode-cathode circuit upon discharge of capacitor 22 through diode 74. The excitation current applied to the gate is also discontinued at this time so that the rectifier will not again begin conducting until such time as the entire circuit is again readied for a new firing cycle. It will be appreciated that the capacitor 79 and inductance 77 result in conserving electrical energy from the power supply and thus reduce the drain on the vehicle battery 26.

What I claim as my invention is:

1. A capacitor-discharge ignition system for internal combustion engines comprising a step-up transformer having primary and secondary windings, a capacitor in series with the primary winding, a DC power source to charge the capacitor in one polarity, first discharge means to discharge the capacitor in only one direction through the primary winding in timed sequence with an operating engine to induce a voltage in the secondary winding suflicient to fire a spark plug, second discharge means bypassing the primary winding to discharge the capacitor in only the other direction when it is charged in another polarity as a consequence of collapse of the magnetic field in the transformer, said second discharge means comprising a diode connected across the primary winding from a point between one side of the capacitor and one side of the winding to the other side of the winding, said other side of the winding being connected to ground, a second diode connected between ground and the other side of the capacitor, and an LC circuit comprising parallel connected inductance and capacitance in series with said second diode between said second diode and ground operable to recharge said capacitor back to said one polarity after discharge of said capacitor through said second discharge means.

2. A capacitive-discharge ignition system for internal combustion engines comprising a step-up transformer having primary and secondary windings, a capacitor in series with the primary winding, a DC power source to charge the capacitor in one polarity, first discharge means to discharge the capacitor in only one direction through the primary winding in timed sequence with an operating engine to induce a voltage in the secondary winding sufficient to fire a spark plug, said first discharge means including a solid-state controlled rectifier having a gate, an anode and a cathode, the anode-cathode circuit of the rectifier connected across the capacitor and primary winding, a signal source in the gate-cathode circuit to provide an excitation current in timed sequence with an operating engine sufiicient to cause conduction through the anode-cathode circuit, and a second discharge means bypassing the primary winding to discharge the capacitor in only the other direction when it is charged in another polarity as a consequence of collapse of the magnetic field in the transformer, said second discharge means including a diode connected across the primary winding from a point between one side of the capacitor and one side of the winding to the other side of the winding, said other side of the winding being connected to ground, a second diode connected between ground and the other side of the capacitor, and an L-C circuit comprising parallel connected inductance and capacitance in series with said second diode between said second diode and ground oper- 5 6 able to recharge said capacitor through said second dis- 3,372,681 3/1968 Phillips et a1 123148 charge means. 3,377,998 4/1968 Adams et a1. 123-148 References Cited UNITED STATES PATENTS LAURENCE M. GOODRIDGE, Prlmary Examiner 2,980,093 4/1965 Short. 5 US. Cl. X.R. 3,312,860 4/1967 Strum 315223 315209 3,383,556 5/1968 Tarter 315-209 

